The FBI is investigating a report that lead Sept. 11 hijacker Mohamed Atta asked a lot of “crazy” questions about a Tennessee chemical plant and a dam between two nuclear-power plants after he flew over them last spring.

The strange incident is particularly disturbing in light of revelations that Atta’s right-hand man, Marwan Al-Shehhi, insisted on flying over chemical plants during training flights in Germany.

Atta and a cohort had just landed in the mining town of Copperhill when they started grilling pilot Dan Whitener.

“So, tell me about this chemical plant I just flew over,” Atta asked, Time magazine reports.

The future mass murderer was referring to Boliden Intertrade, which produced sulfuric acid and sulfur dioxide until last year – and he also wanted to know about the dam on the Hiwassee River, which runs between two nuke plants.

“He asked a lot of really crazy questions,” said Whitener, who informed the feds of the encounter after Atta was fingered in the Sept. 11 terror strikes.

A German aviation instructor said last week that Al-Shehhi signed up for a flying course in Bonn in 1999 and surveyed the Bayer and Hoechst plants, along with the Defense Ministry headquarters.

Atta, Al-Shehhi and 17 fellow hijackers perished with thousands of innocents in the suicide attacks on the World Trade Center and the Pentagon – but authorities believe other terrorists are at large.

Bolstering that theory is evidence from “technical sources” that leaders of Osama bin Laden’s al Qaeda terror movement made at least four calls from Afghanistan to the United States after the atrocities, Newsweek magazine reports.

FBI agents believe that bin Laden was trying to activate cells of terrorists waiting for their orders here.

But so far, that investigative thread hasn’t yielded much.

In other developments:

* Authorities foiled four bombing plots in U.S. buildings in Paris, Turkey and Yemen and in a NATO building in Belgium since Sept. 11, sources told the AP.

* Spanish authorities have traced two calls Atta received in July to Madrid – where members of an Algerian extremist group linked to bin Laden were busted last month for plotting suicide attacks.

* More FBI agents were headed for Las Vegas, where probers are especially interested in Atta’s visits to a Vegas Internet cafe.

The group, busted in April, was reportedly tied to a man involved in an aborted attack on the U.S. Embassy in Rome and one of the accused terrorists in a plot to blow up Los Angeles International Airport.

* The FBI is hunting for a Saudi pilot who bought two small planes in Tennessee and vanished shortly before Sept. 11, but probers say it has nothing to do with the hijackings.